You know that subjects and verbs must agree in number. Thus, this is correct:

     The president is heading to Camp David for the weekend.

The singular subject, president, agrees in number with the singular verb is. That’s simple enough, but confusion may arise if too many words are between the subject and verb.

     The hammers in the toolbox in the garage are missing.

At first glance, it may appear that the preceding sentence contains a subject/verb mismatch—the garage are missing.

But a second look assures us that the sentence is correct. You can determine the subject by removing the prepositional phrases in the toolbox and in the garage. Then we have:

     The hammers are missing.

Here’s another example:

     A flock of geese are in the garden.

That sentence seems to make sense, but it’s grammatically incorrect. The subject is flock, not geese. If we remove the prepositional phrase of geese we have:

     A flock is in the garden.

Compound subjects

Subjects with more than one part, which are called compound subjects, cause more confusion. Here are the rules for dealing with them:

When subjects are joined by or, nor, either, or neither the verb agrees with the word closest to it.

     Neither the president nor any of our three vice presidents are available.

Even though president is singular, vice presidents is plural and therefore we use the plural are.

If a compound subject is connected by and then it’s plural.

     The cat and the dog are fighting again.

If the subject parts refer to the same entity, or form a single unit, use a singular verb.

     My friend and colleague Fred is willing to help.

     Peanut butter and jelly is Sandy’s favorite lunch.

(As an interesting aside, I’m writing this article using Microsoft Word, and Word insists that the verbs in the preceding sentences should be are. Word is wrong. The point is you can’t trust computer software to tell you correct word use; there’s no substitute for human knowledge and insight.)

Other situations

Some words that appear to be plural are singular.

     The news is on at 6:30 p.m.

It doesn’t matter that news ends with an s. It’s singular.

Other words are singular but take a plural verb.

     The scissors are in the top drawer.

Scissors are a singular noun, but are is a plural verb.

Writing around the problem

As noted earlier, some of these constructions sound wrong, even though they’re correct. So the best solution is to write around them.

If your sentence is “The herd of cows is/are ready for milking” you can instead write:

     The cows are ready for milking.

or

     The herd is ready for milking.

And if your sentence is “Either dress slacks or a skirt is/are required for admission to the seminar” you can write:

     Admission to the seminar requires either dress slacks or a skirt.

Note that this solution also kills the weak verb is and saves a word.